I do not think clamping the lines would help diagnose the issue, unfortunately. On the engine side, there’s a purge valve that is supposed to be open at certain times and closed at other times. On the fuel tank side, there’s a vent valve that’s supposed to be closed at certain times and open at other times. So, if you block either side off, you’ll get some sort of code when the computer commands the valve open. The computer will assume the valve is remaining shut when it should be open.